Easy. Ha. Once you get all the six flowers and open the door you need to kill the Phase 3 Algos (final form) again, and revisit the hospital for a final sequence and trophy. As with the base game: Go for protection and weapon-damage. The tower is actually pretty giving with health upgrades and weapons and I also got tons and tons of one-hit-shield items. For me the best weapon is the Electric Pylon Driver, paired with the Disgorgers, boss phases are a thing of mere seconds. You can do this.